The invention belongs generally to a class of hydraulic power transfer units and particularly relates to such units having pistons mechanically arranged in a fixed phase relationship with each other to transfer energy from one hydraulic system to another without transferring fluid therebetween. Such hydraulic power transfer units find particular application in connection with modern aircraft where they are used to provide hydraulic system load sharing to maintain system redundancy particularly during takeoff and landing. In the past it has been conventional practice to install multiple and separate hydraulic systems to provide enough redundancy so that the failure of one or more components in a system does not cause catastrophic problems to the aircraft. Each separate system is usually powered from different prime movers, such as from different aircraft engines or auxiliary power units. It is particularly important during takeoff and landing that such redundancy be provided so that the failure of one hydraulic system does not disrupt some of the services required to be operated such as landing gear retraction which requires a large amount of hydraulic energy. To protect against this contingency, transfer motor pumps have been installed between hydraulic systems to permit the transfer of hydraulic energy from one system to another in either direction without the transfer of hydraulic fluid between the systems. This latter requirement that no fluid be transferred between systems assures that the operable system is protected from fluid loss and contamination.
A typical power transfer unit used heretofore in large aircraft consists of two conventional rotary pumps connected together, usually on a single shaft. Unfortunately, this type of motor pump is highly inefficient with very poor performance. Hence these motor pumps generate a substantial amount of heat which must be dissipated by additional equipment. Furthermore, this equipment is relatively noisy which, in some instances, is distressful to some of the passengers and, due to poor performance, cannot be placed passively between two hydraulic systems so that the equipment is immediately available for use. The noise and heat must be accommodated during landing and takeoff but at other times the heretofore conventional units are switched off. This is undesirable because additional complexity must be added to the circuitry to automatically transfer energy from one hydraulic system to another thereafter.
U.S. Pat. No. 3,890,064 entitled "Reciprocating Transfer Pump," of which the present applicant is a co-inventor, provides an improved power transfer device. However, this improved device requires relatively complex valving to effect power transfer in a given direction and to effect an automatic change of direction of the power transfer upon the loss of pressure in one system. In addition, the unit is heavy and considerable complexity is required to remove the significant pressure ripple generated during the valving cycle of the pump. This pressure ripple is undesirable in that it tends to over stress the surrounding hydraulic system. Hence, it is evident that there is still a further need for improved power transfer units to overcome these disadvantages.
There has also been a need for improvement in the coupling between the nutater and pistons in nutater-type pumps and motors. Previously, a certain amount of slop had to be tolerated between the spider and the piston. Such devices, where the relative positioning of the spider and pistons is not positive, are not practical in the present invention as they cannot endure the stressful environment of an aircraft hydraulic system while keeping the valving precise so that the efficiency of the power transfer remains high.
The start-up friction of a motor pump device usually is high due to pressure loads across the valving means thereof. This high start-up friction is undesirable since in aircraft applications, once the pressure in one system has dropped a relatively small amount, it is desirable that hydraulic energy be transferred thereinto and heretofore proposed valving schemes having flat plate valves have resulted in differential pressures in the range of 1200 to 1500 psi (8000 to 12000 kPa) between systems to start the transfer of energy or a very complex mechanism to balance the forces in the valve means.